Motor vehicle alignment systems are important for ensuring that the alignments of wheels on a vehicle are within the specifications provided by motor vehicle manufacturers. If the wheels are out of alignment, there may be excessive or uneven wear of the tires and vehicle suspension system. In addition, the performance of the vehicle, particularly handling and stability, may be adversely affected if the wheels are not properly aligned. As used herein, the term “wheel” or “vehicle wheel” refers to the tire and wheel assembly found on a motor vehicle. Such an assembly generally includes a conventional tire that is mounted on a metal wheel or “rim.”
The wheels of a motor vehicle may be aligned in a number of ways. For example, an operator or an alignment technician can use a vision imaging system such as a computer-aided, three-dimensional (3D) machine vision that employs optical sensing devices, such as cameras, to determine the positions of targets attached to the wheels of a vehicle. Examples of such apparatus and methods are disclosed in U.S. Pat. No. 5,724,743, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels” issued to Jackson, et al. on Mar. 10, 1998; U.S. Pat. No. 5,535,522, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels” issued to Jackson, et al. on Jul. 16, 1996; and Published U.S. patent application No. U.S. 2003/0065466A1, published Apr. 3, 2003, to Jackson, et al. and entitled “Self-Calibrating, Multi-Camera Machine Vision Measuring System”. All three of these patents and patent application are assigned to the assignee of the present invention and incorporated herein by reference. The Visualiner 3D3 Arago™ Aligner (Model # EEWA533A), for example, is a vision imaging alignment system that is available from the John Bean Company of Conway, Ark. (wwwjohnbean.com).
During an alignment process, a number of parameters are measured and adjusted to achieve proper wheel alignment. These parameters can include, for example, camber, caster, steering axis inclination (SAI), toe, turning angle, thrust line, scrub radius and setback.
Camber is the angle of the wheel, measured in degrees, when viewed from the front of the vehicle. If the top of the wheel is leaning out from the center of the car, then the camber is positive, if the top of the wheel is leaning in, then the camber is negative. If the camber is out of adjustment, it will cause tire wear on one side of the tire's tread.
Caster is the angle of a steering pivot that the front wheels turn on, measured in degrees, when viewed from the side of the vehicle. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if the top of the pivot is leaning toward the front of the car, then the caster is negative. If the caster is out of adjustment, it can cause problems in straight line tracking.
SAI is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. This angle causes the vehicle to lift slightly when the wheel is turned away from a straight ahead position, so that the weight of the vehicle causes the steering wheel to re-center when let go after making a turn. If the SAI is different from side to side, the SAI will cause a pull at very slow speeds. Included angle is the angle formed between the SAI and the camber.
The toe measurement is the difference in the distance between the front of the tires and the back of the tires. The toe is usually set close to zero which means that the wheels are parallel with each other. Toe-in means that the fronts of the tires are closer to each other than the rears. Toe-out is just the opposite. An incorrect toe will cause rapid tire wear to both tires equally. When a vehicle is turned, the inner front wheel must toe-out more than the outer front wheel. Measurement of toes during turning is referred to as turning angle.
Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. This distance must be exactly the same from side to side or the vehicle will pull strongly at all speeds. Setback is when one front wheel is set further back than the other wheel. Thrust angle is the direction that the rear wheels are pointing in relation to the center line of the vehicle. If the thrust angle is not zero, then the vehicle will “dog track” and the steering wheel will not be centered. Steering center is simply the fact that the steering wheel is centered when the vehicle is traveling down a straight and level road.
A typical alignment procedure utilizing a vision imaging system includes first attaching targets to all four wheels of the vehicle. Then, individual wheel positioning is done on all four wheels, and target information is collected on all four wheels.
Sometimes, however, it may be desirable to simply conduct an alignment procedure on the two front wheels of a vehicle. For example, if the vehicle is large, such as a truck, bus or mobile home, or if portions of the vehicle's body would obstruct the viewing of targets on the rear wheels, then it may be desired to conduct the alignment procedure on just the two front wheels of the vehicle.
Based on the foregoing, there is a clear need in this field for a new and improved method for measuring and adjusting the alignment of the wheels of a motor vehicle utilizing a vision imaging system. In particular, what is needed is a new and improved method for measuring and adjusting the alignment of the wheels of a motor vehicle utilizing a vision imaging system that allows an alignment technician to only conduct an alignment procedure on the two front wheels of the vehicle if desired.